Low-pressure sputter deposition is widely used for the deposition of thin films. The low background pressure of approximately one millitorr (0.13 Pascal) or less is important to minimize the collisions of sputtered particles with molecules of the background gas as they travel from the sputter target to the deposition substrate. Minimizing these collisions is important both for the cleanliness of the deposited film and avoiding degradation of the sputtered-particle energy, which is important for obtaining good adhesion and high density for the deposited film.
A closely related prior art is the sputter deposition from a grounded target that results from an energetic beam of ions being directed against that target. The most widely used ion source for such an application is the gridded ion source described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745, incorporated herein by reference. Although it would be less likely, a gridless ion source could also be used. The end-Hall type of gridless ion source is described in U.S. Pat. No. 4,862,032—Kaufman, et al., while the closed-drift type of gridless ion source is described in U.S. Pat. No. 5,359,258—Arkhipov, et al., both of which are incorporated herein by reference. The primary advantage of such sputter deposition apparatus is the low pressure that is possible at the deposition substrate. The high pressure required for the generation of ions is confined to the inside of the ion source. The total gas flow is thereby reduced, compared to having the entire volume within the vacuum enclosure at high pressure, and moderate pumping permits the deposition substrate to be maintained at a low background pressure.
There are problems with the prior art of sputter deposition from a target using an energetic beam of ions against that target.
One problem is that the ion beam generated by the ion source must be directed only at the sputter target. Even with carefully machined and expensive ion optics grids, it is common for some energetic ions to strike other hardware besides the target and thereby introduce contamination directly into the deposited film, or through contamination sputtered onto the target, introduce contamination indirectly into the deposited film.
Another problem is the reflection of energetic neutrals from the sputter target. Energetic ions become neutralized in striking the target, and are reflected diffusely to strike the thin film being deposited on the substrate. These collisions with the substrate introduce damage sites in the deposited film.
Yet another problem is the reduced ion current capacity of ion optics for gridded ion sources at energies low enough to minimize the above problem of energetic neutral reflection. As described in the above article by Kaufman, et al., in the AIAA Journal, the ion current capacity of these ion optics varies approximately as the three-halves power of the voltages. Operation at low ion voltages—and energies—therefore severely restricts the ion beam current and thus the process rate.
A related problem is the large gas flow required to operate a gridded ion source when the source must be large to offset the reduction in ion current capacity due to operating the source at low voltages.
In summary, complicated and expensive apparatus is required for sputter deposition with energetic ion beams. Attempts to reduce the damage due to energetic neutrals by reducing the ion energy can result in an increase in the size of the ion source used which, in turn, can result in the increase of the gas flow and a need for larger, more expensive vacuum pumps.
A more recent technology is described in copending application Ser. No. 09/471,662 filed on Dec. 24, 1999, which is expressly incorporated herein by reference. The contamination in the deposited film is reduced in this more recent technology by using an ion source that generates a low-energy ion beam, wherein the energies of ions in this beam are sufficiently low to minimize or eliminate any sputtering due to collisions of these ions with ground-potential surfaces. The ion energy required to sputter the target is obtained, not by acceleration to a high energy in the ion source, but by biasing the target negative relative to ground potential—defined as the potential of the surrounding vacuum enclosure. This approach has a significant advantage over other ion-beam deposition processes in that it is not necessary to confine the ion beam to the sputter target to achieve a high degree of purity for the deposited films.
Because of the negative bias of the target, the secondary electrons emitted from the target are accelerated to a high energy in the plasma sheath adjacent to the sputter target. In this manner, secondary electrons emitted from the target become energetic electrons after being accelerated through the plasma sheath. Under the proper operating conditions, these energetic electrons form a directed beam away from the target. Depending on the locations of the deposition substrate, this beam of energetic electrons could strike, or not strike, the surface of the deposition substrate upon which the film was being deposited.
Energetic electrons are known to cause a variety of effects, both adverse and beneficial, on films deposited on deposition substrates, as described by Ball in J. of Applied Physics, Vol. 43 (1972), beginning on page 3047, and by Chapman, et al., in J. of Applied Physics, Vol. 45 (1974), beginning on page 2115, both of which are incorporated herein by reference. There has been, however, no recognition that specific operating parameters could control the formation, or lack of formation, of a well-defined electron beam by the acceleration of secondary electrons from a sputter target.
Electron-beam sources have been studied separately, as described by Kaufman, et al., in J. of Vacuum Science and Technology, Vol. A3 (1985), beginning on page 1774. An electron beam in a plasma background has an upper limit on electron current beyond which the beam becomes unstable and is scattered diffusely. This maximum electron current for a well-defined beam depends both on the operating parameters and on the specific electron beam geometry.
Using stability limits from electron source technology, the stability of an electron beam formed by secondary electrons can be determined, permitting the directing of the electron beam formed by a biased target either toward or away from deposition substrates. Conversely, it is possible to operate at conditions resulting in electron beam instability, so that the energetic electron beam is scattered broadly, thereby reducing both the total energy and current density of electrons arriving at a particular substrate.
Depending on the substrate and the film being deposited upon it, the beam of energetic electrons could have beneficial or detrimental effects. Examples of the beneficial effects are: the energetic electrons can be used to enhance chemical reactions at the substrate surface, to increase nucleation of a deposited film, as well as to drive an electrically floating substrate surface to a negative potential and thereby enhance the bombardment of the substrate surface by low-energy background ions, thereby modifying film stress or producing increased film adhesion, density, or refractive index. Examples of detrimental effects are: the energetic electrons can damage sensitive film and substrate materials, either directly by collision damage or overheating, or indirectly by electrostatic charging. Knowledge of the presence and location of this energetic electron beam, which depends in turn on being able to determine its stability or instability, thus permits the use of this electron beam in deposition when the effects are beneficial and the avoidance of this electron beam when the effects are detrimental.